Sky Guy: Changing seasons? It's simple geometry

“But Tom,” you are asking, “why are we cheering? What is the Vernal Equinox?”

Great questions indeed, and hence the subject for today’s effort.

In astronomy, an equinox refers to one of two points where the celestial equator intersects with the ecliptic.

Whoa! That sounds like a lot of astronomical mumbo-jumbo! But don’t stop reading yet. Let’s see if we can sort through this. As the scientific method dictates, we begin by making some observations.

You have probably observed that the sun seems high in the sky during the summer and low in the sky during the winter. This is because Earth’s rotation axis is tipped with respect to our orbital plane around the sun, about 23.5 degrees.

So, as Earth orbits the sun, there is a time when one hemisphere is tilted toward the sun and the other away. It is this tilt and the transition from one extreme to the other that gives us our different seasons.

Consider: the Earth is actually farther from the sun during summer than it is during winter. So it is not the proximity to our star that makes the summers hot. Rather, our hemisphere is tilted toward the sun, giving us longer and hotter days.

As the Earth travels in its orbit, the sun is constantly in motion, moving either up or down in latitude throughout the year. In the Northern Hemisphere, when the sun has reached its highest point, which is over the Tropic of Cancer, it is the first day of summer, called the Summer Solstice.

Similarly, six months later, when the sun is at its lowest point, over the Tropic of Capricorn, it is the first day of winter — the Winter Solstice.

The equinoxes are the two points in between the solstices when the sun is directly over the equator. Here, the lengths of day and night are about 12 hours each. In fact, the word “equinox” itself means “equal night.”

Imagine we are standing on the equator. On the first day of spring, the Vernal Equinox, the sun would be directly overhead at noon. The sun then creeps a little bit farther north with each passing day.

Three months later, the sun would be 23.5 degrees north of the equator. It is the Summer Solstice, around June 21. In the months that follow, the sun moves a little farther south each day.

On about Sept. 21, the Autumnal Equinox, the sun is again directly over the equator. The sun continues its southerly trek.

Finally, on Dec. 21, the sun is at its lowest point in the sky, over the Tropic of Capricorn. It is the Winter Solstice.

The sun continues this path in our sky, going from one Tropic Circle to the other and back again, every year, year after year. And although we are observing the perceived motion of the sun in our sky, called the ecliptic, we must remember that it is the Earth that is moving, not the sun.

And now our earlier definition makes sense. The celestial equator is merely a projection of Earth’s equator into space and the ecliptic is the apparent path of the sun, so the equinoxes are the two points where these great circles intersect — either the Vernal or Autumnal Equinox.

Now, you’re saying, “Tom, I get it. What do I do?

Nothing, really. These are geometric alignments, not “events” to stay up and “look” for.

Until next time, keep looking up!

Tom Webber, the Times-Union’s Sky Guy, is the director of the Bryan-Gooding Planetarium/Alexander Brest Space Theater at the Museum of Science & History. He writes a column each month in the Life section and also lends his expertise when there’s something special going on, or just when he feels like it.

WHAT’S UP IN MARCH

After all the cold weather that has besieged Jacksonville, March brings us the beginning of spring and soothing thoughts of warmer days ahead. But even though the nights are getting shorter, we shouldn’t stop turning our attention to starlit nights.

The most famous constellation of them all, Orion the Hunter, is found on the meridian right after sunset, recognized by his three belt stars that are in the center of a bow-tie-shaped asterism (connect-the-dot picture). The southwestern star in this asterism is Rigel, and the northeastern star is Betelgeuse (pronounced “Beatle-Juice!”).

Just above Betelgeuse is the mighty planet Jupiter, the largest planet in the solar system. Not counting our moon, Jupiter will be the brightest object in the evening sky.

For you early risers, there are two special planetary treats this month. Mercury will be at Greatest Western Elongation on March 14, meaning it will be the farthest from the sun from our terrestrial point of view. On March 23, Venus will be at Greatest Western Elongation.